Some men rest on their laurels. Physicist Richard Garwin rests on his mistakes. In the 1980s, Mr. Garwin derided Ronald Reagan's plan to build a space-base defense against Soviet missile attacks. Mr. Garwin's "case" against the Strategic Defense Initiative relied heavily on calculations that quickly proved erroneous, and other assertions later discredited. At the time however, Mr. Garwin and the Union of Concerned Scientists, declared SDI too expensive, infeasible and likely to fuel an accelerated arms race. Sound familiar?

Today, Mr. Garwin and many of his same friends in the scientific community rank among the most influential opponents of the proposed National Missile Defense System, a largely land-based system. In letters to President Clinton, congressional testimony and "scientific" reports about missile defense, they offer much the same pseudo-scientific arguments as they did against "star wars." These ostensibly objective scientists will likely continue to influence the ongoing debate, no matter which party finally captures the White House.

After all, you can argue with Ted Kennedy, but to dispute Mr. Garwin or his colleagues is to question Science. Or is it? The reality is that Mr. Garwin and Co. made a number of gross errors back in the 1980s about SDI everything from how many satellites would be required to the total cost ($3 trillion when under $100 billion was more likely). The errors of the UCS are long forgotten, but they shouldn't be especially as Americans today again worry about the dangers of rogue missiles or terrorist attacks.

Before anyone else mistakes this bunch for reliable experts, it is worth considering how their previous assertions collided with reality.

Fresh on the heels of President Reagan's proposal for SDI in 1983, the UCS estimated that to adequately ensure against Soviet missile attacks would require 2,400 satellite battle stations at a possible total cost of $3 trillion. It turned out the actual number of satellites needed was closer to 100 the total cost, $100 billion, according to government scientists. Why the discrepancy? In a December 1984 Commentary magazine article, Dartmouth physicist Robert Jastrow explained that the UCS' wildly inflated estimates for SDI's total cost stemmed from a rather simple error; the UCS had underestimated the range of each satellites orbit. Each satellite, in short, could protect against far more Soviet missiles than the UCS thought.

Faced with criticism, the Union of Concerned Scientists backtracked again and again. In what became known as the "Garwin curve," the UCS repeatedly changed its official estimates throughout 1984 and 1985. The magic number went from 2,400 to 800 then down to 300, The Washington Post reported in 1985.

Others folks called the program unworkable. Why? The UCS had certainly provided many specifics. Mr. Garwin and like-minded naysayers insisted that key components of the SDI program including computer technology and chemical lasers were utterly infeasible. How did these assertions withstand the test of time?

One supposedly intractable problem was computer programming. SDI's critics claimed the system would require "10 million lines of computer code, and [therefore] could not possibly be written in a way that makes it functional," recalls William Graham, President Reagan's science adviser. Sounds foreboding. But Mr. Graham says today much bigger codes are "used in everyday applications." A typical office computer can handle about 100 million lines of code, he says.

What about chemical lasers, which Richard Garwin and the UCS said would require too much fuel and offer too little firepower for an effective defense?

Early this June, the jointly developed U.S.-Israeli Tactical High Energy Laser (THEL) tracked and destroyed a live Katyusha rocket. In late August, another test had the laser successfully take down two rockets in rapid succession.

Hindsight isn't always 20/20. Outside the Vatican, it is hard to find anyone possessed of a greater doctrine of infallibility than Richard Garwin. Interviewed by the American Spectator in late September, Mr. Garwin insisted all his calculations about SDI were entirely correct. But professor, why did so many people find errors? "I don't have time to talk to you. I'm too busy."

Curiously, many of the other folks who peddled junk science alongside Mr. Garwin claim the battle over SDI somehow vindicates their current position. Cornell University physicist Kurt Gottfried, who with Mr. Garwin overestimated the number of satellites needed for a defense, lately has inveighed against proposals for a National Missile Defense (NMD) system.

In April, Mr. Gottfried, now chair of the Union of Concerned Scientists, wrote the introduction to a UCS report that purported to discredit NMD. His battle-weary tone suggested a frustrated teacher who just can't get his dim-witted charges to see the light. "The scientific evidence we present to you is clear the system will not work." In the end, these views carried the day. President Clinton this September decided to postpone deployment of the NMD.

Declaring that the the system as a whole is not yet proven, Mr. Clinton punted. He decided to let his successor make a final decision as to how or if the nation should proceed. Whoever finally assumes the presidency will need to be as nonpartisan as possible. To that end, the supposedly objective analysis from the UCS could prove even more influential. Some men look to the heavens and say "why not." Mr. Garwin and other scientists continue to say definitely not. Politicians faced with these unrepentant naysayers would do well to reply, "Oh no, not you again."

Evan Gahr is a senior fellow at the Hudson Institute. He adapted this commentary from his longer article in the November issue of the American Spectator.